Various types of energy sources consisting of photocells activated by some type of nuclear radiation are known in the prior art. These devices, sometimes referred to as "nuclear batteries" or "atomic batteries", convert nuclear electromagnetic radiation into electrical energy by one of two methods, single conversion systems or double conversion systems. Single conversion nuclear batteries generate electrical energy by converting the nuclear radiation (i.e. alpha particles, beta particles or gamma radiation) into electrical energy by direct absorption of the nuclear radiation at the p-n junction of a semiconductor material, for example, U.S. Pat. Nos. 3,094,634 and 3,304,445. Double conversion nuclear batteries generate electrical energy by converting the nuclear radiation into electromagnetic radiation, usually by irradiating a phosphorescent material that will generate light in the visible spectrum, and then converting that electromagnetic radiation into electrical energy by absorbtion of the electromagnetic radiation at the p-n junction of a semiconductor material, usually a typical photovoltaic cell, for example, U.S. Pat. Nos. 3,031,519, 3,053,927, and 3,483,040.
While the concept of a nuclear battery is not new, a practical and commercially feasible device of this type has not been possible because of the extreme dangers involved in the handling and use of radioactive materials. Most nuclear batteries of the type known in the prior art have either been unsafe or have required such extensive shielding of the nuclear material used to power the battery that the device is rendered impractical for most applications. The regulatory standards for radiation leakage upon container failure impose additional constraints that limit the applications for such devices. One possible means of overcoming these safety limitations is to limit the amount of radioactive material used in such a device. For example, in a typical smoke detector a small amount of radioactive foil containing one microcurie of radioactive Americium 241 is used to power the detection circuit of the device. In general, regulatory standards allow for small amounts of radioactive material to be used under certain circumstances. For example, with proper shielding and packaging, a device containing 5 curies of radioactive material may be approved by the Nuclear Regulatory Commission for limited commerical activities. These low limits on radioactive material effectively limit the radiation energy, and hence, the electrical energy that may be generated from any such source.
Using the amount of radioactivity as measured in curies, the total amount of power available from such an energy source can be calculated. Each curie of radioactive material will produce 3.7.times.10.sup.10 Beqerels (decays)/second. Assuming that the radioactive emission is in the form of a beta particle from the radioisotope tritium having an average 5.6 KeV of energy, the total theoretical power emitted is 32.5 microwatts/curie. Theoretically, if there were a complete conversion of all of the power of this nuclear radiation to electrical energy, the total amount of power available from a small, but safe, amount of radioactive material containing less than 25 curies of tritium would be less than 1 milliwatt. Though the total amount of power generated by such a device over the half life of the tritium radioactive material may be on the order of a hundred watt-hours, until recently relatively few applications could operate with a continuous power supply outputting in the microwatt range. With the advent of CMOS and other low power circuitry, however, applications and uses for this type of long-life, low-watthour power supply are now becoming more practical.
Although a variety of self-luminous, low light sources have been available for a long time (e.g. radium and tritium activated phosphors used for creating self-luminous paints for watch dials, etc., U.S. Pat. Nos. 3,033,797, 3,325,420 and 3,342,743), it has generally been regarded that such materials were unsuitable for commercial use for the conversion of light into electricity. The low levels of radioactivity associated with such materials, though generally not harmful or dangerous, do not provide an adequate source of power for the nuclear batteries of the type known in the prior art. In addition to the low light level (50 micro-lamberts or less), such sources may also be characterized by rapid and unpredictable light decay and, in the case of radium-activated light sources, may produce undesirable radiation hazards associated with their decay products.
Though the concept of a long-life, electrical energy source activated by a radioactive material is attractive and has many potential applications none of the prior art devices have been able to create a safe, yet sufficiently powerful, energy source that is commercially feasible. Accordingly, there is a continuing need to develop a safe and practical long-life, electrical energy source powered by a radioactive source.